1. Field of the Invention
This invention relates to a supporting member for the papermaking screen in paper machines, and more particularly, to a member for contiguously supporting the papermaking screen which is running continuously, said member being designed to lessen the abrasion of the screen by being made of a sintered body consisting mainly of silicon nitride of porosity less than about 0.8%.
2. Prior Art
FIGS. 1(a) and (b) show schematic drawings of prior art paper machines to which the member according to the present invention has been applied, and FIG. 2 shows a longitudinal sectional view giving an example of where the member according to the invention is applied to the forming board.
We will here describe the outline of the prior art paper machines, and at the same time point out their deficiencies, with reference to these figures.
Of the different paper machines which utilize the member according to the present invention with the object of supporting the papermaking screen, the one with a long screen type as shown in FIG. 1(a) is so designed that the paper material, namely, slurry consisting of paper pulp and water is supplied from a head box 11 onto the screen (principally made of metal) 12 which is being driven by rollers 18, 19 in the direction of the arrow shown in the figure, and the watery paper pulp 13 is dehydrated progressively with the aid of the running of the screen 12 and the tractive force of rollers 22, 23 which are positioned just under the screen 12, then being followed by the treatment of rolling, drying, and so on.
For the purposes of partly aiding the dehydration of the pulp and partly supporting the screen 12, the construction of the paper machine of such a type is bolstered by a forming board 14, a deflector 15, a foil 16, and a suction box 17, which all are arranged contiguously just under the screen 12 in succession, and further by being provided with tension rollers 20, 21.
In addition to this type, there is also used another kind of paper machine, that is, a twin wire type such as is shown in FIG. 1(b). This type is so designed that the two screens 12, 12' are driven by a driving roller 18 and tension rollers 20, 21, while paper pulp 13, after having been supplied from the head box 11 which is set up above, is compressed from both sides between the two screens 12, 12' which are supported by the supporting members 24. The pulp is dehydrated by the suction box 17, and then the resulting dehydrate is treated by rolling and drying.
In these paper machines, there are provided supporting structures such as the forming board 14, the foil 16 and others which are used, for example, as in the one shown in FIG. 1(a), for the purpose of contiguously supporting the screen 12. As part of these structures, such as the forming board 14, as shown in FIG. 2, a plurality of supporting members 14' are arranged to extend transversely to the running direction of the screen 12. In such supporting members, there has been widely used heretofore a ceramic material obtained from a sintered body of alumina or silicon carbide or others, producing somewhat satisfactory results. But with these conventional members there has resulted a large degree of abrasion of the screen 12 running incessantly over the supporting members 14', though the wear rate depended on the property of paper pulp.
On the other hand, in the latest paper machines higher running speeds have been used. Therefore, with the use of an apparatus which comprises a conventional forming board, deflector, table roll, and suction box, the required dehydration cannot take place to the fullest extent, resulting in the deterioration of quality of the paper produced.
As a countermeasure, there is now being used a foil in the capacity of a means for both supporting the screen and dehydrating. However, using prior art materials which are quite porous, this means will be eroded due to abrasion. Thus the efficiency of dehydration drops, which effects a bad influene on the quality of paper. This also occurs with the supporting member being provided on the suction dehydrator, such as a suction box. In addition, the supporting members heretofore in use, such as are made of alumina ceramic, silicon or the like, can also detrimentally abrade the screen itself to a great degree.